This invention relates to a permanent magnet of a samarium(Sm)-cobalt(Co) system, particularly to a permanent magnet of Sm.sub.2 Co.sub.17 system. More particularly, it relates to a permanent magnet having excellent magnetic properties such as residual magnetic flux density (Br), coercive force (.sub.I Hc) and maximum energy product ((BH)max), and also excellent oxidation resistance.
As a permanent magnet of R--M system (wherein R is a rare earth element such as Sm, Ce or Y; and M is Co or such a metallic element as Cu, Fe, etc. to be used in combination with Co), there has been proposed a variety of permanent magnets having various composition. For these permanent magnets, maximum energy product ((BH)max) and residual magnetic flux density (Br) are particularly important properties when they are used for motors and the like; the values of these properties are desired to be as large as possible. However, it is difficult to enhance these values unless the coercive force (.sub.I Hc) of the magnet is larger than a certain value. Accordingly, it becomes necessary to increase the .sub.I Hc value in order to obtain a permanent magnet having larger values of (BH)max and Br.
In the case of a permanent magnet of Sm.sub.2 (Co, Cu, Fe, Ti).sub.17 -system, it is known that Br value can be increased by increasing Fe content or by decreasing Cu content. However, Br or (BH)max of the magnets cannot be improved by merely increasing Fe content or by simply decreasing Cu content, since increase of Fe content or decrease of Cu content results in a lowering of .sub.I Hc value. For this reason, the composition of the conventional Sm.sub.2 (Co, Cu, Fe, Ti).sub.17 -system magnet has been determined with the aim of making Br value as large as possible while maintaining .sub.I Hc value at a level larger than a certain value.
For instance, Japanese Patent Publication No. 15096/1980 discloses that a permanent magnet prepared by molding in a magnetic field a powdered alloy composed of 10 to about 30% by weight of Y and other rare earch element(s), 0.2.about.7% by weight of Ti, 5.about.20% by weight of Cu, 2.about.15% by weight of Fe, and the balance of Co principally followed by sintering the same, is excellent in oxidation resistance and also in magnetic properties such as .sub.I Hc and (BH)max. Further, Japanese Laid-Open Patent Application No. 109191/1977 discloses a permanent magnet prepared by molding in a magnetic field a powdered alloy composed of 23 to about 30% by weight of Sm, 0.2 to about 1.5% by weight of Ti, 9 to about 13% by weight of Cu, 3 to about 12% by weight of Fe, and the balance of Co principally. These prior art magnets, however, can not necessarily be considered to be satisfactory ones, since the composition of these magnets has resulted from a compromising adjustment between the changes of residual magnetic flux density (Br) and coercive force (.sub.I Hc) which are caused by varying Cu content and Fe content.
A permanent magnet having excellent magnetic properties, i.e., large Br value and (BH)max value, will be obtainable if it becomes possible to reduce the Br-lowering Cu component, increase the Br-enhancing Fe component and, at the same time, maintain .sub.I Hc value being at a level higher than a certain value.